In such an engine, the combustion chamber delimited by the upper part of the piston and the cylinder head contains a fuel mixture that can undergo combustion under the effect of a spark ignition device, such as a spark plug.
It has been observed that this combustion is the source of various combustion noises some of which can seriously damage the engine.
A first noise is the background noise whose vibrational frequency is of very low amplitude and nearly constant. This noise is the result of the propagation of the combustion, which is normally propagated with a flame front from the spark plug. This type of noise is sufficiently low not to risk damaging the engine.
A second noise, which is a parasitic noise, is the result of an unwanted combustion referred to as engine knock in the combustion chamber. This engine knock is a sudden and localized self-ignition of part of the fuel mixture prior to the arrival of the flame front due to the ignition of the fuel mixture by the spark plug. As it is generally known, this engine knock produces vibrations of higher amplitude than the background noise, and at given frequencies. This engine knock leads to a local pressure increase and can generate, in case it occurs repeatedly, destructive effects on the engine, especially at the piston.
Finally, a third noise is generated by abnormal combustion. This abnormal combustion mainly affects engines whose size and/or cylinder capacity has been reduced while keeping the same power and/or the same torque as conventional engines, commonly referred to as downsized engines. This type of engines, mainly of gasoline type, is greatly supercharged and this abnormal combustion generally occurs at low engine speeds. In fact, in such an operating range, timing of the fuel mixture combustion is far from the optimum. More precisely, ignition occurs once the piston is beyond its top dead center (TDC). This allows to have a pressure curve referred to as “double-hump curve”, where the first hump is the result of compression and of the descent of the piston after the TDC, followed by another hump resulting from the ignition, by the spark plug, of the fuel mixture and from the combustion thereof throughout the expansion phase in the combustion chamber. However, considering the high pressures and temperatures reached in the combustion chamber by supercharging, abnormal combustion takes place long before ignition of the fuel mixture by the spark plug. This combustion is the result of self-ignition with a flame front of a large part of the fuel mixture in the vicinity of the piston TDC and it occurs well upstream from the ignition of the fuel mixture by the spark plug. This abnormal combustion produces a low noise referred to by specialists as rumble type combustion.
All that has been written so far is illustrated by FIG. 1, which is a graph comprising a curve Np showing the evolution of the pressure (in bar) in the combustion chamber as a function of the crankshaft angle (in°) during normal combustion and a curve Rp showing the evolution of this pressure as a function of the same crankshaft angle during rumble type combustion. This evolution is considered during a phase of the piston stroke going from the compression bottom dead center (compBDC at 180° crankshaft) to the expansion bottom dead center (expBDC at 540° crankshaft).
Thus, for normal combustion (curve Np), i.e. without engine knock or rumble, the fuel mixture is compressed in the combustion chamber to about 50 bars as the piston moves from the compression bottom dead center (compBDC) to the vicinity of the top dead center (TDC at about 360° crankshaft angle). From this TDC, the piston stroke is in the opposite direction and the pressure decreases to about 40 bars at a crankshaft angle Va of the order of 380°. At this angle Va, ignition of the compressed fuel mixture is achieved by the spark plug. This therefore provides combustion of the fuel mixture and a pressure increase to about 60 bars at a crankshaft angle of approximately 405°. The piston continues its descending motion until it reaches the expansion BDC and the pressure decreases until it is close to the atmospheric pressure.
For a rumble type combustion (curve Rp), considering the pressure and temperature conditions of the fuel mixture in the combustion chamber, self-ignition of this mixture occurs before the piston reaches the TDC. This self-ignition generates a very large and sudden pressure increase in the combustion chamber, the pressure exceeding 140 bars, long before ignition of the fuel mixture by the spark plug. This pressure then decreases as a result of the piston stroke from the TDC to the expBDC thereof, until it reaches the level of the atmospheric pressure in the vicinity of the expBDC.
This rumble type combustion thus leads to very high pressure levels in the combustion chamber, which can produce partial or total destruction of the moving elements of the engine, such as the piston or the connecting rod, the engine being consequently out of order.
The present invention thus aims to identify an abnormal rumble type combustion with the devices and systems commonly used in engines so as to be able to take the steps allowing to prevent such a combustion during subsequent engine running.